First of all, a multi-antenna system for performing precoding using feedback information is explained as follows.
FIG. 1 is a block diagram of a multi-antenna system.
FIG. 1 is showing a configuration of a transmitting end of the a multi-antenna system and the transmitting end could be a base station or a UEuser equipment (UE). In the transmitting end, e.g. the base station, user data to be transmitted to a receiving end is inputted as a single stream or a multiplexed data stream to a channel encoder 101.
The channel encoder 101 performs channel encoding on the inputted single stream or the inputted multiplexed data stream.
A modulator 102 generates symbolized data by performing constellation mapping on a channel encoded signal.
The symbolized data are multiplied by a precoding matrix 103 and then transferred to the corresponding antennas 104, respectively.
Meanwhile, the receiving end transmits feedback information for the precoding matrix 103. In other words, the precoding matrix 103 used for precoding is preferably selected by the feedback information from the receiving end for the precoding matrix 103. And, a controller 105 of the transmitting end may select user or precoding matrix 103 using the feedback information.
To estimate MIMO (multi input multi output) channel, the receiving end transmits a non-precoded common pilot signal via the respective antennas of the transmitting end, e.g. the base station.
In the general communication technologies proposed by 3GPP LTE (long term evolution), e.g. PARC (per antenna rate control), PSRC (per stream rate control) and PU2RC (per user unitary rate control), the multi-antenna system can be implemented with the configuration shown in FIG. 1 as well.
The 3GPP LTE introduces a precoding scheme as a closed loop multi-antenna. Representatively, there are PU2RC, S-PUSRC (SIC-based per user and stream rate control), and the like.
In case of PU2RC, Fourier based matrix which may be expanded according to the number of transmitting antennas is used as a unitary precoding matrix.
            e      m              (        g        )              =                            1                      M                          ⁡                  [                                    w                              0                ⁢                m                                            (                g                )                                      ⁢                                                  ⁢            …            ⁢                                                  ⁢                          w                                                (                                      M                    -                    1                                    )                                ⁢                m                                            (                g                )                                              ]                    T        ,          ⁢            w      nm              (        g        )              =          exp      ⁢              {                  j          ⁢                                    2              ⁢              π              ⁢                                                          ⁢              n                        M                    ⁢                      (                          m              +                              g                G                                      )                          }            
In Formula 1, em(g) indicates a unitary precoding matrix and ‘M’ indicates a total number of transmitting antenna and ‘G’ indicates a total group number of precoding matrixes. ‘n’ indicates an index of nth antenna and ‘g’ indicates an index of gth group. So, it is able to specify a precoding matrix using ‘n’ and ‘g’. And, ‘m’ indicates an mth virtual beam forming pattern.
In case of S-PUSRC, a switching beam forming vector is used as a precoding matrix.P=[a1a2a2N] a1=[1ejφt . . . ej(N-1)φt]T φi=kd sin(θi)  [Formula 2]
In Formula 2, ‘N’ indicates a total number of antenna, ‘ai’ indicates a ith precoding matrix, ‘k’ indicates a wavelength, ‘θ’ indicates a steering direction, and ‘d’ indicates a distance between antennas neighboring to each other.
The closed-loop multi-antenna system using the precoding schemes can support one of SDM (space division multiplexing) scheme for transmitting at least one data stream for a single user, SDMA (space division multiplexing access) scheme for transmitting data via a specific beam for each of multiple users, and a beam forming scheme for transmitting data by forming a specific beam. In the following description, the above three schemes are named, single-user MIMO, multi-user MIMO, and beam forming, respectively.
In the closed-loop system, in case of the single-user MIMO, accuracy varies according to a data quantity of antenna weight that is, feedback information from a receiving end (e.g., UE) to affect MIMO performance. In particular, if the number of antennas is equal to or greater than 4, a size of a codebook is raised. So, a data quantity of feedback information increases.
In case of the multi-user MIMO, if many precoding matrixes are used for a codebook design, it is difficult to perform grouping for multi-users. So, it is difficult to configure the MIMO system itself.
In case of the beam forming, an inter-antenna interval of the beam forming is different from that of the MIMO scheme. So, the transmitting system is unable to adopt both of the beam forming scheme and the MIMO scheme simultaneously.